I use my REX a bit different then most..
when ever I get up on the expressway and the traffic flow is fast (70+) I turn on the REX to save battery for the Destination.
This comes in Handy on my monthly trip to Milwaukee, I am generally at about 75% battery by the time I hit Gurnee, so I can turn it on until I get to Milwaukee, do my errands, head back home and turn the REX back on.
Generally I can make it to Milwaukee and Back on one charge and 1 tank of Gas. with room to spare, while traveling the with the flow of traffic.
I haven't had the Car through the winter yet, so I assume I will have to either charge or refuel during that trip in the cold months.
This is a trip that was a total pain in the ... with my Ford Focus Electric.. with that car I would have to top off my charge in Gurnee (about an hour) charge in Milwaukee (about 2 hours ) then charge a little back in Gurnee (about an hour) adding about 4 hours to a two and half hour (driving) trip.
I hope BMW continues the REX concept, nothing is faster then having your charger on board and able to use it in real time even if it is gas powered.